video sync errors and missing stream after clipping h264 files

Description

Summary of the bug:
If I re-encode a file with h264, any later attempts to clip/trim/resize that file result in various sync errors or even a missing video stream.

How to reproduce:
After starting with a raw camera file, after I process it by resizing
and/or cropping etc, using the h264 codec, When I later try to use that
output file as an input file for further processing, (like to trim
the length and starting time) it gives an error and drops the video stream, or it gets the video way out of sync.
Looks like the same bug, with different starting times specified.

I suspect a problem with tbc and tbn, watch the input and output values.
Step 1, resize only:

This results in a very out of sync video. The time marker starts at 4 seconds, or specifically A: 0.0 V: 4.4
and the video only lasts 4 seconds, as the audio and video arrive at 8 seconds separately. Also the footsteps are clearly out of sync. The video stops while the audio continues to play the correct clip starting and ending points.

Since this did not duplicate my earlier report (Jan22 on ffmpeg-users), I adjust the starting sec to reproduce it.

Also the tbc don't match. Step 1 wrote the tbc as 30 (default), it becomes 60 (default) at the 2nd stage input, and then 15360 (default) at the last output.
Which tbc is (default)? The meaning of default is lost and confusing.

Error message: tbc=15360/1 invalid seems like perhaps it is confusing tbc and tbn?

In the real projects I'm working on, (not just this test video), I also see the bug I'm trying to report.
I finally see a pattern. When a h264 video has larger I-frame intervals, like 2 or 2.5 seconds, if I try to do just the clipping -ss # -t # -c copy (before or apart from any other work, like crop/resize/watermark/etc,) then I see mis-matched aud/vid sync way too often, as I rarely request the video to be cut at exactly the I-frame position.
The same happens after ffmpeg -c:v h264 Any subsequent attempts to clip the video results in messed up aud/vid sync. The key is when the I-frame interval is other than 0.5 or 1.00 second, the usual clipping locations.

Ok I've now worked this through enough times to see this may not be a true bug after all, but a limitation on the I-frame location relative to the desired clipping spots.

Maybe the only realistic improvement would be to add a warning that the first x.x seconds were lost before the first I-frame was found.